Electrically-actuated devices are widely employed with many types of wireline tools which are selectively actuated from the surface. One of the common uses for such electrically-initiated control devices is to selectively actuate explosive devices on such typical well tools as perforating guns, cutting tools, dump bailers, sample takers and backoff tools which are dependently supported in a borehole or well bore by so-called "wireline" or a suspension cable having electrical conductors connected to a surface power source. Once the tool is positioned at a desired depth location in a well bore, the power source is operated for supplying power to an electrically-actuated detonating system on on the tool for setting off the explosive devices on the tool.
Although there is a wide variety of wireline tools equipped with electrically-actuated detonating systems and various types of explosive devices, these detonating systems are fairly similar inasmuch as they are basically comprised of a blasting cap or an electrically-responsive initiator or detonator having a sensitive primary explosive, such as lead azide, cooperatively arranged to set off a powerful secondary explosive, such as RDX, which, in turn, detonates the explosive devices on the tool. For example, a typical wireline perforator has an electrically-responsive detonator which is cooperatively arranged to set off a first explosive device such as a booster charge or a detonating cord. This first explosive device is, in turn, arranged in detonating proximity of one or more second explosive devices such as shaped explosive charges which are appropriately mounted on the tool for perforating the well casing and earth formations.
Premature actuation of any of these wireline tools must, therefore, be avoided if possible.
One of the most common sources for the premature actuation of wireline tools with electric detonators is, of course, the careless application of power to the cable conductors after the well tool is connected to the suspension cable and the tool is still at the surface. To at least minimize these risks, the installation of the detonators into the tool as well as the final connection of their electrical leads will be delayed as long as is reasonably possible. Added protection is also provided by controlling the surface power source with a key-operated switch that is not unlocked until the tool is situated at a safe depth.
Perforators have also been protected heretofore by one or more switches in the downhole electrical firing circuit of the perforator which will not be closed until a movable member on the perforator body has been moved outwardly against the adjacent casing wall. Other prior-art disarming devices have included pressure-actuated switches in the downhole firing circuit which are not activated until the perforator is safely disposed at a depth where the downhole pressure is greater than a predetermined level. Another prior-art protective device is described in U.S. Pat. No. 3,517,758 as having an arming switch in the downhole firing circuit which has a spring-biased movable contact member that is initially held in an inactive position by a destructible link such as a typical carbon resistor. Then, when it is desired to activate the arming circuit for a perforator, a current of a predetermined magnitude is passed through the carbon resistor for a sufficient length of time to weaken or destroy the resistor and thereby release the spring-biased switch contact for movement so as to activate the downhole electrical circuit of the perforator.
These procedures will, of course, greatly reduce the hazard of inadvertently detonating the explosive devices in these tools while they are at the surface. Nevertheless, a major hazard is that the electrical detonators commonly used for oilfield explosive tools are susceptible to being inadvertently detonated by strong electromagnetic fields. Another source of premature actuation of these detonators is also the unpredictable presence of so-called "stray voltages" which may sporadically appear in the structural members of the drilling platform. Such stray voltages are not ordinarily present; but these voltages are frequently created by power generators on the drilling rig, cathodic protection systems for the structure or galvanic corrosion cells that may be present at various locations in the structure. Lightning also may set off these detonators. At times there may be hazardous voltage differences existing between the wellhead, the structure of the drilling rig and the equipment used to operate the tools.
Many of these hazards may also be present when a well tool having an unfired detonator and one or more unexpended explosive devices is removed from the well bore. This situation itself represents an additional hazard since it is not always possible to know if there is an unexpected electric detonator remaining in that tool. It should be appreciated that everyone who is in the vicinity will be well aware of the potential danger when any tool with explosive devices is being retrieved. Thus, even low-order detonation of explosive devices on a tool being retrieved can be a problem since nearby personnel will overreact to the sudden noise and possibly injure themselves or damage equipment as they are seeking safety.
Because of these potential hazards that exist once a tool is armed, many proposals have been made heretofore for appropriate safeguards and precautions for handling these tools while they are at the surface. For instance, when a tool with an electric detonator is being prepared for lowering into a well, in keeping with the susceptibility of detonators to strong electromagnetic fields it usually necessary to maintain strict radio silence in the vicinity. Ordinarily a temporary restriction on nearby radio transmissions will not respresent a significant problem on a land rig. On the other hand, when service tools with explosives are being used on a drilling vessel or an offshore platform, it is a common practice to at least restrict, if not prohibit, radio and radar transmissions from the platfrom as well as from helicopters and surface vessels in the vicinity. Similarly, it may be best to postpone electrical welding operations on the rig or platform since welding machines may develop currents in the structure that may inadvertently initiate a sensitive electrical detonator in an unprotected well tool at the surface.
It will, of course, be realized that a large amount of time is frequently lost when a well tool having electrically-actuated explosive devices is being prepared for operation since ancillary operations that are not related to the service operation are also typically curtailed. For example, the movements of personnel and equipment by helicopters and surface vessels must be restricted to avoid radio and radar transmissions which might set off one of the detonators. Thus, the relative priorities of the proposed service operation and these movements must be taken into account to decide which operations must be curtailed in favor of the higher-priority tasks. These problems relating to operations on one drilling rig may also similarly affect operations on other offshore rigs in the vicinity. Accordingly, if there are a large number of platforms or drilling vessels situated in a limited geographical area, the activities on all drilling rigs in that area must be coordinated to accommodate the service operations in the affected area. The various delays and the related logistical problems associated with these activities will, of course, have an obvious effect on the expenses and the time requirements for the drilling and service operations in that particular field.
In view of these problems, various proposals have been made heretofore to disarm a well tool by temporarily interrupting the explosive train at some point between the initiating explosive device and the other explosive devices. One of the most common proposals has been to position a removable detonation-attenuating barrier between the initiator or donor explosive of the detonator and the adjacent receptor explosive. The detonator is thereafter armed by simply removing the barrier just before the well tool is lowered into the well bore. Other proposed disarming devices employ an axially-movable or a rotatable barrier that is normally positioned to interpose a solid wall between the spaced donor and receptor explosives. To arm the tool, the barrier is manipulated for moving an opening in the barrier into alignment with the explosives. It must be appreciated, however, that despite the effectiveness of these barriers, once a barrier is moved the well tool becomes vulnerable to inadvertent actuation.
Regardless of the particular wireline tool that is involved, it must be realized that even when the tool is in a well bore and thereby shielded against radio or radar signals, an electric detonator can be inadvertently fired should sufficient voltage be prematurely applied in any way to the conductors in the tool suspension cable.
Outside of the oilfield industry, other disabling devices have also been proposed heretofore in such diverse fields as fire sprinklers and military ordinance. These devices typically have various arrangements of spring-loaded detonating pins which are releasably retained in ineffective positions by explosive squibs, bimetallic elements or an erodible retainer that is responsive to prolonged exposures to a corrosive environment. However, these devices simply have little or no application for oilfield tools. For instance, the delay times for these disabling devices are either so short or unpredictable that they would be unusable in the oilfield. Other devices have been proposed to utilize electronic compensators to accommodate for the variations in the surrounding environment. Those latter devices are typically so imprecise and delicate that they would be totally inadequate for selectively controlling electrically-actuated explosive devices in a wireline tool.
Impact-detonated tools have also employed barriers of frictionally-insensitive inert materials such as consolidated talc and the other materials listed in U.S. Pat. No. 2,857,847 to limit the penetration of a firing pin through the barrier. It is, of course, readily recognized that those barriers are used only for controlling the depth of penetration of the firing pin and are effective immediately without any reference whatsoever to the ambient temperature of the barrier material.
Despite the large number of different safety procedures and devices described above, these systems are still incapable of reliably and safely operating well tools that have explosive devices that are to be actuated by means of remote electrical or mechanical means. Instead, it has been necessary to rely upon complicated fail-safe actuators with the unwarranted expectation that all of the fail-safe devices on a particular tool will always properly perform to prevent the malfunction of the tool during a given operation. Moreover, none of these safety devices are completely suited for use with different types of well tools. The safety devices which are currently in use are substantially tailor-made for a particular tool; and, therefore, few of those prior-art safety devices or electrically-actuated detonating systems can be utilized on other types of tools without extensive redesign or modification of a particular detonating system or its related tool. It also appears that none of these prior-art systems have had at least two fail-safe features which is wholly fool-proof and reliable.